CAPE TOWN, Feb 3 (Reuters) - Prominent South African activist Mamphela Ramphele has backtracked on a week-old pledge to run for president for the main opposition Democratic Alliance, frustrating the party's push to win more black votes in an election just months away.

A physician, author and former partner of the late activist Steve Biko, Ramphele said last week she would challenge President Jacob Zuma as the DA's candidate, a potential turning point for a party keen to ditch its image as a guardian of white privilege.

But Monday's about-face could spell the end of Ramphele's career as a credible challenger to the ruling African National Congress, coming just days after she was accused of opportunism for defecting to the DA from her own fledging party.

Ramphele said on Monday her decision to defect to the DA had been rushed and without consultation of some of her own party members, meaning she would likely face a fierce backlash from her support base.

"The time for this was not right," she told a news conference, adding she would stay with her Agang party, which has gained little traction since its founding.

Veteran political commentator Allister Sparks said the DA had time to regroup before the vote.

"I think it puts a fatal end to Mamphela Ramphele's image and reputation as a political figure," he said.

"It will do much less damage to the DA - it was an attempt to burnish its image by having a black face there - but we've still got about three months to go before the election and I think there's time to recover."

Democratic Alliance leader Helen Zille denounced her old friend for reneging on the election deal so swiftly. "Dr Ramphele has demonstrated once and for all that she cannot be trusted to see any project through to its conclusion," she said.

The African National Congress has ruled South Africa since the first democratic elections in 1994 and is expected to cruise to victory again in parliamentary polls scheduled to be held by May.

Yet the DA is hoping to tap into the increasing number of blacks frustrated by the slow pace of change and growing perceptions of corruption.

While ANC support has waned amid the corruption scandals and charges it has failed to rescue millions of blacks from poverty, it still won nearly two-thirds of the vote in the last election in 2009 and its overall majority this year is not in question.

Ramphele has so far retained the respect of much of the country's black majority due to her relationship with Biko, who died in 1977 from beatings by apartheid police.

She was also placed under house arrest by the apartheid government because of her political work. (Writing and additional reporting by David Dolan; Editing by Alison Williams)